Serum response factor (SRF) and Myocardin (MYOCD) constitute a molecular trigger switch for the activation of a battery of vascular smooth muscle cell (VSMC) cyto-contractile and ion channel genes containing SRF- binding CArG boxes. We first showed that levels of MYOCD (but not SRF) correlate with the degree of VSMC differentiation. Since the VSMC differentiation program is not fixed and subject to adaptation in a number of disease processes where levels of MYOCD change, we have been interested in understanding the regulation of MYOCD expression and its function as a homeostatic switch for the preservation of a quiescent, contractile state in VSMC. Accordingly, we have reported in a series of papers that MYOCD (a) is sufficient for conferring VSMC contractile competence through SRF-dependent changes in contractile and ion channel gene expression; (b) completely represses the program of skeletal muscle differentiation; (c) controls a growing number of microRNAs, including the microRNA143/145 gene; and (d) is positively induced by TGF1 through a p38MAPK-dependent pathway. Preliminary data further demonstrate cis elements for a transcriptional repressor and new microRNA as well as functional data showing, for the first time, the favorable effects of MYOCD expression on vascular occlusive disease, lipid uptake, and inflammatory marker expression. Collectively, our growing body of work serves as a critical foundation to test the hypothesis that MYOCD is a homeostatic switch for normal VSMC differentiation. This thesis will be tested through a series of inter-related specific aims designe to elucidate transcriptional and post-transcriptional control of MYOCD expression and the role of MYOCD in vascular remodeling during injury-induced neointimal expansion and atherogenesis. In Aim 1, bacterial artificial chromosome transgenic mice will elucidate the function of novel cis-acting elements leading to activation or repression of MYOCD. In Aim 2, novel MYOCD loss- and gain-of-function mice will directly assess the role of this powerful cofactor in experimental vascular disease processes, including effects on VSMC inflammatory, proliferative, and transdifferentiative states. In Aim 3, genomic studies integrating human and mouse CArGome data we have generated (> 84,000 CArG boxes) with RNA-seq of VSMC where MYOCD is expressed in the absence or presence of SRF will be carried out. Such an analysis will reveal a subset of the CArGome that is responsive to MYOCD as well as new SRF-independent MYOCD target genes of import in vascular disease. Thus, the planned studies will yield new insight into the in vivo regulation of MYOCD expression during normal postnatal development and in vascular disease processes as well as novel information related to MYOCD as a likely inhibitor of vascular remodeling associated with physical injury and atherosclerotic disease. The results of these studies will have enormous applications for devising new therapeutic strategies to combat acute and chronic vascular diseases and perhaps other diseases where MYOCD expression/activity may be altered (e.g., asthma, hypertension, Alzheimer's disease).